The invention relates to a plasma display comprising a front panel, a rear panel and, arranged therebetween, a number of gas-containing plasma cells which are separated from each other by partitions, in which plasma cells a plasma may be formed, in a plasma region, between two discharge electrodes.
Such a plasma display is known, for example, from EP 764 965 A2. Such a plasma display customarily comprises a matrix of plasma cells (microcavities) in which a gas discharge is ignited. This gas discharge preferably generates radiation in the UV range, which radiation is converted by a phosphor layer present in the plasma cell into visible red, green or blue light. This visible light can be transmitted to the exterior through the transparent glass front panel.
Apart from the high manufacturing cost and the expensive driver electronics for the high-voltage drive, the low efficiency, particularly the very low discharge efficiency, is regarded to be a drawback of such plasma displays.
Therefore, it is an object of the invention to provide a plasma display with an improved discharge efficiency and a higher efficacy. In accordance with the invention, this object is achieved by the plasma display described in claim 1.
The high losses in known plasma displays can be attributed, in particular, to the fact that after the ignition of the gas discharge, a layer is formed in the vicinity of the discharge electrode acting as a cathode, which layer is commonly referred to, in the case of glow discharges, as cathode trap. In the region of this layer facing the cathode, a very high electric field strength in combination with a low ion and electron density is observed. In said region, the current is carried, in particular, by the ions which outnumber the electrons. As a result of the high electric field strength, ions in this region are accelerated substantially and release their energy through elastic collisions to the gas molecules and the walls.
The inventive means for locally narrowing the plasma region are suitably provided at locations where there is a high electron density, i.e. not in the direct vicinity of the cathode. By narrowing the plasma region, a region having a high field strength is generated in which the electrons are accelerated. Thus, in a region having a high electron density, also the average electron energy levels are high, so that in this region electric energy is efficiently converted to excitation energy and hence radiation energy. In this region, a quasi-neutral state again prevails, the current flow, however, being predominantly carried by the electrons. Consequently, a greater proportion of the available power is coupled into regions having a high efficiency, so that the overall efficacy of the plasma display is increased.
The object in accordance with the invention is achieved also by a plasma display as claimed in claim 2. By extending the discharge path (i.e. the path where the discharge between the discharge electrodes takes place) between the discharge electrodes, it is achieved that the cathode range referred to as cathode trap, in which the number of electrons and ions are approximately equal, becomes larger relative to the other regions between the discharge electrodes. Consequently, the zone which is subject to losses becomes relatively smaller. As a result, UV radiation can be generated more efficiently and the losses occurring in the cathode trap in front of the cathode are smaller.
The inventive solutions as claimed in claim 1 and 2 are based on the idea in accordance with the invention that an increase of the discharge efficiency and a higher efficacy can be achieved by providing means which bring about that in a region between the discharge electrodes the electric field is as strong as possible and that said region contains as many electrons as possible, so that as many electrons as possible can be excited.
The invention is preferably employed in AC plasma displays, in which the plasma cells are driven by an alternating voltage, and in which the discharge electrodes are covered, as claimed in claim 4, with a dielectric layer. The invention can in principle also be used however in DC plasma displays in which the discharge electrodes are not covered with a dielectric layer.
The advantageous further embodiments of the invention as claimed in claims 5 and 6 constitute simple solutions which, dependent upon the location where they are applied and their dimensions, may bring about both a local narrowing of the plasma region and an extension of the discharge path.
In other types of plasma displays, in which a discharge electrode is arranged on the front panel as well as on the rear panel, the means for narrowing the plasma region may, as claimed in claim 7, also take the form of a diaphragm arranged at the partitions for separating the individual plasma cells from each other.
Since, in AC plasma displays the symmetry of the discharge with regard to the polarity, i.e. the similarity of the plasma near the cathode and the anode, is very important, said means are preferably centrally arranged between the discharge electrodes, as claimed in claim 8. This does not affect the symmetry. It is also feasible, however, to deliberately use plasma-asymmetry, and deliberately arrange the means asymmetrically.
Preferably, the means used for narrowing are made of a dielectric material, as claimed in claim 9. However, it is alternatively possible to use other materials, such as metal or metal with a dielectric coating, thus enabling the means for narrowing or path extension to be given a fixed potential.
The inventive embodiment as claimed in claim 10 is very easy to manufacture and adjust. If the recesses are suitably embodied, as claimed in particular in claim 11, it is even possible to provide a number of narrowed portions in the plasma region and simultaneously extend the discharge path.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.